1. Field of the Invention
The present invention relates to an engine intake system, and more particularly, to an intake system of a fuel injection type engine. More specifically, the present invention pertains to an engine intake system having an intake passage separated into a primary passage and a secondary intake passage, the latter being provided with a shut-off valve which is adapted to be closed under a light load engine operation so that the intake air is drawn to the combustion chamber only through the primary passage under the light load engine operation.
2. Description of the Prior Art
In the field of the engine intake system, there have been various proposals for providing improvements in respect of pollutant emissions and fuel economy. One of such proposals is to divide the intake passage into a primary passage of a relatively small cross-sectional area and a secondary passage of a relatively large cross-sectional area. The secondary passage is provided with a shut-off valve which is adapted to be closed under a light load operation of the engine so that the intake air is drawn into the combustion chamber only through the primary passage under the light load engine operation. Since the primary passage is of a relatively small cross-sectional area, the intake air flow speed can be maintained relatively high even though the quantity of the intake air may be small under light load.
As an example, in the Japanese patent application No. 58-176776 which has been filed on Sept. 24, 1983 and disclosed for public inspection on June 5, 1985 under the disclosure No. 60-101224, there is disclosed an engine intake system having an intake passage which is divided into two branch passages respectively connected with two intake ports opening to the same combustion chamber. Beneath one of the branch passages there is formed an auxiliary passage of a smaller cross-sectional area opening to one of the intake passages. The intake passage is provided with a shut-off valve for closing the intake passage under a light load engine operation. The auxiliary passage is opened to the intake passage upstream of the shut-off valve. It is therefore considered that the auxiliary passage is the primary passage whereas the branch passages constitute the secondary passage.
As regards the fuel supply system, proposals have also been made for a so-called lean mixture combustion system so that improvements are accomplished in respect of fuel economy and pollutant emissions. Such lean mixture combustion system is aimed to provide a stratified mixture charge to the combustion chamber so that a relatively rich mixture is formed in the vicinity of the ignition plug to thereby avoid ignition failure whereas the overall mixing ratio can be made leaner than in a conventional engine. With this type of fuel supply system, it is possible to decrease the amount of pollutant emissions in the exhaust gas. For accomplishing the stratified mixture charge, a preferred fuel supply system is so-called fuel injection system wherein the engine intake passage is provided with a fuel injection valve.
The fuel injection system includes such a type wherein fuel is injected into the intake passage irrespective of the operating cycle of the engine. Another type is the one wherein the fuel injection timing is synchronized with the engine operating cycle such as the one which is disclosed by the Japanese patent application No. 55-184692 filed on Dec. 24, 1980 and disclosed for public inspection on July 6, 1982 under the disclosure No. 57-108428. This type may be called as the "timed injection type" and includes the fuel injection valve for each cylinder. The fuel injection valve may be actuated for a predetermined time period in each cycle of the engine operation for example at the end of the intake stroke. This system is preferable in that a uniform fuel distribution can be ensured among the combustion chambers. the system is further preferable in that the stratified mixture charge can be conveniently accomplished.